Vascular graft prostheses are widely used in a variety of surgical procedures. Grafts may be used to replace sections of natural blood vessels which have been removed either through accident or through surgery. Grafts might also be used to provide an alternative route for blood, e.g., to bypass an especially occluded portion of an arteriosclerotic vessel, or might be used to form a new blood pathway in vascular reconstruction procedures.
The most common type of vascular prosthesis is a fabric graft, which may be either woven, knitted or velour. The earliest grafts were generally woven; however, later it became accepted that knitted grafts had advantages over woven grafts for many applications. Particular advantages which have been ascribed to knitted grafts include their high porosity, softness and flexibility and the potentially high velour index of their surfaces, which was felt to promote healing through endotheliazation, i.e., ingrowth of tissue. It is found, however, that endotheliazation occurs to a considerably lesser degree in vascular prostheses implanted in humans than was previously predicted, and thus, certain of the advantages that were previously ascribed to knitted prostheses have not been realized. In fact, it has been found that the high velour surfaces of knitted grafts may simply act to collect dead tissue, particularly on their interior surface.
Woven grafts have certain recognized advantages over knitted grafts. They can be made with a porosity which is lower than the porosity available in knitted prostheses and can be constructed with greater fabric uniformity, e.g., their porosity is more uniform. In the weaving process, the weft thread is mechanically compacted and pushed against the adjacent weft thread, leaving either substantially no pores or very small pores. In certain instances, such as where the patient has been administered anticoagulants or for certain types of vascular repair, a prosthesis through which blood does not seep at all is required, and for such applications the graft should have a very low porosity which is only available in a woven graft.
Where pores do exist in either a knitted or a woven fabric, which would otherwise allow blood to seep through, the pores are generally closed by preclotting. The patient's own peripheral blood is drawn, the graft is spun in the patient's blood to uniformly soak the graft and the blood-drenched graft is exposed to air for a short period of time which is sufficient for fibrin to build up within the pores and close the pores. Even for applications where preclotting may be used, a porosity lower than that which is generally attainable in a knitted graft is desirable to insure fluid integrity of the graft, and in this respect, woven grafts have an important advantage over knitted grafts. Woven grafts are therefore returning to favor for use in applications where knitted grafts had recently been preferred.
Typically, a graft is woven in a tubular configuration in which the weft or pick is actually a single long thread shuttled helically through a two-layer array of warp threads, as described in U.S. Pat. No. 3,316,557, forming a flat tubular configuration. The tubular configuration is then opened to a cylindrical configuration, set and crimped. One problem that is still presented by woven grafts is their tendency under certain conditions to unravel. A vascular graft is usually supplied with excess length and is subsequently cut by the surgeon. Whereas a knitted graft can be knitted in a manner that substantially eliminates unravelling after it is cut, a woven graft is typically left with at least one end that is partially frayed due to cutting. A frayed fabric end of a woven graft may promote progressive unwinding of the helically wound weft thread, resulting in eventual weakening of the graft. This fraying problem has tended to limit the use of woven vascular grafts even when their low porosity and other advantages, such as higher mechanical strength, would otherwise indicate their use. It would be advantageous to have a woven graft which is not subject to unravelling.